diff --git a/CMakeLists.txt b/CMakeLists.txt
index 1156aee..151433b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -48,6 +48,7 @@ add_library(secsgem
   src/secsi/header.cpp
   src/secsi/block.cpp
   src/secsi/protocol.cpp
+  src/secsi/tcp_transport.cpp
   src/gem/control_state.cpp
   src/gem/communication_state.cpp
   src/gem/process_job_state.cpp
@@ -85,6 +86,7 @@ add_executable(secsgem_tests
   tests/test_hsms.cpp
   tests/test_hsms_connection.cpp
   tests/test_secsi.cpp
+  tests/test_secsi_tcp.cpp
   tests/test_control_state.cpp
   tests/test_communication_state.cpp
   tests/test_data_model.cpp
diff --git a/COMPLIANCE.md b/COMPLIANCE.md
index e6b3c15..fbaa007 100644
--- a/COMPLIANCE.md
+++ b/COMPLIANCE.md
@@ -54,7 +54,8 @@ Legend:
 | T3 reply timer                        | ⬜     | E4 §10.1 | Driven by the upper layer (same as HSMS). |
 | T4 inter-block timer                  | ⬜     | E4 §10.1 | Multi-block message-gap; FSM emits hook events. |
 | Master/slave contention resolution    | ✅     | E4 §7.1.4 | Slave yields on simultaneous ENQ; master holds. |
-| Serial port wiring (asio)             | ⬜     | —        | FSM is IO-free; a follow-up commit will land the asio integration. |
+| Serial port wiring (asio)             | ⬜     | —        | FSM is IO-free; serial integration is a wiring follow-up. |
+| TCP tunnel for testing                | ✅     | —        | `secsi::TcpTransport` wraps the FSM behind an asio TCP socket; mirrors secsgem-py's `secsitcp/`. |
 
 ---
 
diff --git a/include/secsgem/secsi/tcp_transport.hpp b/include/secsgem/secsi/tcp_transport.hpp
new file mode 100644
index 0000000..b7819b1
--- /dev/null
+++ b/include/secsgem/secsi/tcp_transport.hpp
@@ -0,0 +1,75 @@
+#pragma once
+
+#include <asio.hpp>
+#include <functional>
+#include <memory>
+#include <string>
+#include <vector>
+
+#include "secsgem/secs2/message.hpp"
+#include "secsgem/secsi/block.hpp"
+#include "secsgem/secsi/header.hpp"
+#include "secsgem/secsi/protocol.hpp"
+
+// SECS-I block framing tunneled over a TCP socket.  Mirrors the
+// secsgem-py `secsitcp/` convenience layer — useful for back-to-back
+// testing and simulators where you want SECS-I semantics (ENQ/EOT/ACK,
+// retries) but no actual serial hardware.  The on-wire bytes are
+// identical to what would travel over RS-232; only the carrier differs.
+namespace secsgem::secsi {
+
+class TcpTransport : public std::enable_shared_from_this<TcpTransport> {
+ public:
+  using MessageHandler = std::function<void(secs2::Message)>;
+  using ErrorHandler   = std::function<void(const std::string&)>;
+  using LogHandler     = std::function<void(const std::string&)>;
+
+  TcpTransport(asio::ip::tcp::socket socket, Role role, uint16_t device_id,
+               Timers timers = {});
+
+  void set_message_handler(MessageHandler h) { on_message_ = std::move(h); }
+  void set_error_handler(ErrorHandler h)     { on_error_ = std::move(h); }
+  void set_log_handler(LogHandler h)         { on_log_ = std::move(h); }
+
+  // Begin the read loop.  Idempotent.
+  void start();
+
+  // Queue a SECS-II message for transmission.  Internally split into
+  // SECS-I blocks if larger than kMaxBlockBody; system bytes are
+  // auto-assigned monotonically.
+  void send(secs2::Message msg);
+
+  // Hard-close the underlying socket.  No graceful teardown — SECS-I has
+  // no equivalent of HSMS Separate.req at the protocol level.
+  void close();
+
+  Protocol::State state() const { return fsm_.state(); }
+
+ private:
+  void read_loop();
+  void run_actions(std::vector<Action>& actions);
+  void deliver_block(Block block);
+  void start_timer(Timer which);
+  void cancel_timer(Timer which);
+  void log(const std::string& msg);
+
+  asio::ip::tcp::socket socket_;
+  Protocol fsm_;
+  Timers timers_;
+  uint16_t device_id_;
+  bool started_ = false;
+  bool closed_ = false;
+
+  std::array<uint8_t, 256> read_buf_{};
+  std::vector<Block> assembler_;        // partial multi-block message
+  uint32_t next_system_bytes_ = 1;
+
+  asio::steady_timer t1_;
+  asio::steady_timer t2_;
+
+  MessageHandler on_message_;
+  ErrorHandler   on_error_;
+  LogHandler     on_log_;
+};
+
+}  // namespace secsgem::secsi
diff --git a/src/secsi/tcp_transport.cpp b/src/secsi/tcp_transport.cpp
new file mode 100644
index 0000000..4b7080b
--- /dev/null
+++ b/src/secsi/tcp_transport.cpp
@@ -0,0 +1,132 @@
+#include "secsgem/secsi/tcp_transport.hpp"
+
+#include <variant>
+
+namespace secsgem::secsi {
+
+TcpTransport::TcpTransport(asio::ip::tcp::socket socket, Role role,
+                           uint16_t device_id, Timers timers)
+    : socket_(std::move(socket)),
+      fsm_(role, timers),
+      timers_(timers),
+      device_id_(device_id),
+      t1_(socket_.get_executor()),
+      t2_(socket_.get_executor()) {}
+
+void TcpTransport::start() {
+  if (started_) return;
+  started_ = true;
+  read_loop();
+}
+
+void TcpTransport::send(secs2::Message msg) {
+  Header tmpl;
+  tmpl.device_id    = device_id_;
+  tmpl.system_bytes = next_system_bytes_++;
+  auto blocks = split_message(msg, tmpl);
+  std::vector<Action> actions;
+  for (auto& b : blocks) {
+    fsm_.on_event(EventSend{std::move(b)}, actions);
+  }
+  run_actions(actions);
+}
+
+void TcpTransport::close() {
+  if (closed_) return;
+  closed_ = true;
+  asio::error_code ec;
+  socket_.shutdown(asio::socket_base::shutdown_both, ec);
+  socket_.close(ec);
+  t1_.cancel();
+  t2_.cancel();
+}
+
+void TcpTransport::read_loop() {
+  if (closed_) return;
+  auto self = shared_from_this();
+  socket_.async_read_some(
+      asio::buffer(read_buf_),
+      [self](std::error_code ec, std::size_t n) {
+        if (ec) {
+          if (self->on_error_ && ec != asio::error::operation_aborted)
+            self->on_error_(std::string("read: ") + ec.message());
+          self->close();
+          return;
+        }
+        std::vector<Action> actions;
+        for (std::size_t i = 0; i < n; ++i) {
+          self->fsm_.on_event(EventByte{self->read_buf_[i]}, actions);
+        }
+        self->run_actions(actions);
+        self->read_loop();
+      });
+}
+
+void TcpTransport::run_actions(std::vector<Action>& actions) {
+  for (auto& act : actions) {
+    if (auto* t = std::get_if<ActionTransmit>(&act)) {
+      auto buf = std::make_shared<std::vector<uint8_t>>(std::move(t->bytes));
+      auto self = shared_from_this();
+      asio::async_write(socket_, asio::buffer(*buf),
+                        [self, buf](std::error_code ec, std::size_t) {
+                          if (ec && self->on_error_ &&
+                              ec != asio::error::operation_aborted) {
+                            self->on_error_(std::string("write: ") + ec.message());
+                            self->close();
+                          }
+                        });
+    } else if (auto* d = std::get_if<ActionDeliverBlock>(&act)) {
+      deliver_block(std::move(d->block));
+    } else if (auto* st = std::get_if<ActionStartTimer>(&act)) {
+      start_timer(st->which);
+    } else if (auto* ct = std::get_if<ActionCancelTimer>(&act)) {
+      cancel_timer(ct->which);
+    } else if (auto* e = std::get_if<ActionRaiseError>(&act)) {
+      if (on_error_) on_error_(e->reason);
+    }
+  }
+}
+
+void TcpTransport::deliver_block(Block block) {
+  assembler_.push_back(std::move(block));
+  if (!assembler_.back().header.end_block) return;
+  auto msg = assemble_message(assembler_);
+  assembler_.clear();
+  if (!msg) {
+    if (on_error_) on_error_("assemble_message: malformed block sequence");
+    return;
+  }
+  if (on_message_) on_message_(std::move(*msg));
+}
+
+void TcpTransport::start_timer(Timer which) {
+  asio::steady_timer* timer = nullptr;
+  std::chrono::milliseconds dur{0};
+  switch (which) {
+    case Timer::T1: timer = &t1_; dur = timers_.t1; break;
+    case Timer::T2: timer = &t2_; dur = timers_.t2; break;
+    case Timer::T3: case Timer::T4: return;  // upper-layer concern
+  }
+  timer->expires_after(dur);
+  auto self = shared_from_this();
+  timer->async_wait([self, which](std::error_code ec) {
+    if (ec) return;  // cancelled or destroyed
+    std::vector<Action> actions;
+    self->fsm_.on_event(EventTimeout{which}, actions);
+    self->run_actions(actions);
+  });
+}
+
+void TcpTransport::cancel_timer(Timer which) {
+  switch (which) {
+    case Timer::T1: t1_.cancel(); break;
+    case Timer::T2: t2_.cancel(); break;
+    case Timer::T3: case Timer::T4: break;
+  }
+}
+
+void TcpTransport::log(const std::string& msg) {
+  if (on_log_) on_log_(msg);
+}
+
+}  // namespace secsgem::secsi
diff --git a/tests/test_secsi_tcp.cpp b/tests/test_secsi_tcp.cpp
new file mode 100644
index 0000000..b33cc4d
--- /dev/null
+++ b/tests/test_secsi_tcp.cpp
@@ -0,0 +1,152 @@
+// Integration test for the SECS-I TCP transport.  Two TcpTransport
+// instances are wired back-to-back over a loopback TCP pair; the test
+// sends a SECS-II message in one direction and asserts the other side
+// reassembles and delivers it.  Verifies that the FSM, the framer, and
+// the asio I/O loop play together end-to-end.
+
+#include <doctest/doctest.h>
+
+#include <asio.hpp>
+#include <chrono>
+#include <memory>
+#include <string>
+#include <thread>
+#include <vector>
+
+#include "secsgem/secs2/item.hpp"
+#include "secsgem/secs2/message.hpp"
+#include "secsgem/secsi/tcp_transport.hpp"
+
+using namespace secsgem::secsi;
+namespace s2 = secsgem::secs2;
+
+namespace {
+
+struct SocketPair {
+  asio::io_context io;
+  asio::ip::tcp::socket a{io};
+  asio::ip::tcp::socket b{io};
+
+  SocketPair() {
+    asio::ip::tcp::acceptor acc(io, asio::ip::tcp::endpoint(
+                                        asio::ip::address_v4::loopback(), 0));
+    const auto port = acc.local_endpoint().port();
+    bool accepted = false, connected = false;
+    std::error_code ec_a, ec_b;
+    acc.async_accept(a, [&](std::error_code ec) { ec_a = ec; accepted = true; });
+    b.async_connect({asio::ip::address_v4::loopback(), port},
+                    [&](std::error_code ec) { ec_b = ec; connected = true; });
+    while (!(accepted && connected)) {
+      if (io.stopped()) io.restart();
+      if (io.poll() == 0) std::this_thread::sleep_for(std::chrono::milliseconds(1));
+    }
+    REQUIRE_FALSE(ec_a);
+    REQUIRE_FALSE(ec_b);
+  }
+};
+
+template <typename Pred>
+void pump_until(asio::io_context& io, Pred pred,
+                std::chrono::milliseconds budget = std::chrono::seconds(5)) {
+  const auto deadline = std::chrono::steady_clock::now() + budget;
+  while (!pred()) {
+    if (std::chrono::steady_clock::now() > deadline) FAIL("pump_until budget exceeded");
+    if (io.stopped()) io.restart();
+    if (io.poll() == 0) std::this_thread::sleep_for(std::chrono::milliseconds(1));
+  }
+}
+
+}  // namespace
+
+TEST_CASE("SECS-I TCP transport: single-block round-trip") {
+  SocketPair sp;
+  auto master = std::make_shared<TcpTransport>(std::move(sp.a), Role::Master, 1);
+  auto slave  = std::make_shared<TcpTransport>(std::move(sp.b), Role::Slave, 1);
+
+  std::vector<s2::Message> received;
+  slave->set_message_handler([&](s2::Message m) { received.push_back(std::move(m)); });
+
+  std::vector<std::string> errors;
+  master->set_error_handler([&](const std::string& e) { errors.push_back(e); });
+  slave->set_error_handler([&](const std::string& e) { errors.push_back(e); });
+
+  master->start();
+  slave->start();
+
+  master->send(s2::Message(1, 13, true, s2::Item::ascii("LOT-99")));
+
+  pump_until(sp.io, [&] { return !received.empty() || !errors.empty(); });
+  REQUIRE(errors.empty());
+  REQUIRE(received.size() == 1);
+  CHECK(received[0].stream == 1);
+  CHECK(received[0].function == 13);
+  CHECK(received[0].reply_expected);
+
+  master->close();
+  slave->close();
+  pump_until(sp.io, [&] { return master->state() == Protocol::State::Idle; },
+             std::chrono::seconds(1));
+}
+
+TEST_CASE("SECS-I TCP transport: multi-block message reassembles correctly") {
+  SocketPair sp;
+  auto master = std::make_shared<TcpTransport>(std::move(sp.a), Role::Master, 1);
+  auto slave  = std::make_shared<TcpTransport>(std::move(sp.b), Role::Slave, 1);
+
+  std::vector<s2::Message> received;
+  slave->set_message_handler([&](s2::Message m) { received.push_back(std::move(m)); });
+
+  std::vector<std::string> errors;
+  auto err = [&](const std::string& e) { errors.push_back(e); };
+  master->set_error_handler(err);
+  slave->set_error_handler(err);
+
+  master->start();
+  slave->start();
+
+  // Build a body that comfortably exceeds kMaxBlockBody.
+  std::string big(700, 'X');
+  master->send(s2::Message(7, 3, true, s2::Item::ascii(big)));
+
+  pump_until(sp.io, [&] { return !received.empty() || !errors.empty(); });
+  REQUIRE(errors.empty());
+  REQUIRE(received.size() == 1);
+  CHECK(received[0].stream == 7);
+  REQUIRE(received[0].body.has_value());
+  CHECK(received[0].body->as_ascii() == big);
+
+  master->close();
+  slave->close();
+}
+
+TEST_CASE("SECS-I TCP transport: bidirectional exchange") {
+  SocketPair sp;
+  auto master = std::make_shared<TcpTransport>(std::move(sp.a), Role::Master, 1);
+  auto slave  = std::make_shared<TcpTransport>(std::move(sp.b), Role::Slave, 1);
+
+  std::vector<s2::Message> at_master, at_slave;
+  master->set_message_handler([&](s2::Message m) { at_master.push_back(std::move(m)); });
+  slave->set_message_handler([&](s2::Message m) { at_slave.push_back(std::move(m)); });
+
+  std::vector<std::string> errors;
+  auto err = [&](const std::string& e) { errors.push_back(e); };
+  master->set_error_handler(err);
+  slave->set_error_handler(err);
+
+  master->start();
+  slave->start();
+
+  master->send(s2::Message(1, 1, true, s2::Item::ascii("ping")));
+  pump_until(sp.io, [&] { return !at_slave.empty() || !errors.empty(); });
+  REQUIRE(errors.empty());
+  REQUIRE(at_slave.size() == 1);
+
+  slave->send(s2::Message(1, 2, false, s2::Item::ascii("pong")));
+  pump_until(sp.io, [&] { return !at_master.empty() || !errors.empty(); });
+  REQUIRE(errors.empty());
+  REQUIRE(at_master.size() == 1);
+  CHECK(at_master[0].function == 2);
+
+  master->close();
+  slave->close();
+}